Zoom lens

ABSTRACT

A zoom lens comprising, from the object side to the image side, a front lens group ( 72 ), a controllable lens group, and a rear lens group ( 74 ), the controllable lens group comprising a voltage-controlled electrowetting device, which device contains a first fluid (A) and a second fluid (B) having different refractive indices, with at least two first fluid-second fluid interfaces ( 40,42 ). The curvatures, and thus the lens power, of these interfaces can be changed independently by supplying a voltage (V 1 , V 2 ) to electrodes ( 22,32 ) of the device, so that no mechanical movement of lens elements is needed.

The invention relates to a zoom lens having at least a first fixed lensgroup, a second fixed lens group, and a controllable lens group.

The invention also relates to a camera comprising such a zoom lens andto a handheld device comprising such a camera.

A conventional zoom lens comprises a number of solid lens elementshaving fixed refractive surface curvatures and made of a transparentmaterial like glass or a transparent plastic. These lens elements aregrouped in a front lens group at the object side, a rear lens group atthe image side, and a controllable lens group between the front groupand the rear group. Each of these groups may consist of one or more lenselements. Lens elements of the controllable lens are movable withrespect to each other for performing zooming and focusing. Zooming isunderstood to mean changing the image scale, i.e. selecting the size ofthe object scene that is imaged, by changing the focal distance of thelens system. The maximum settings of the zoom lens are Teleconfiguration, wherein a small portion of an object scene is imaged, andwide configuration, wherein a larger portion of an object scene isimaged. By moving lens elements of the controllable lens group, the zoomlens can be set between these two extreme configurations andconfigurations therebetween. Focusing is understood to mean keeping theselected object scene in focus for every configuration of the zoom lenssystem.

Because of the required mechanical movement of lens elements, theconventional zoom lens has a large dimension along its optical axis sothat it is less suitable for miniaturization. An electric motor, whichconsumes energy, is used for moving several lens elements. If a zoomlens is to be used in a miniature camera forming part of a small,handheld and battery powered, apparatus such as a mobile phone, the zoomlens should be miniaturized. The conventional zoom lens design is notsuited for such use because the electric motor consumes a considerableamount of battery power and the mechanical vulnerability becomes aproblem. Moreover, mechanical zooming requires a certain amount of time.

It is an object of the invention to provide a zoom lens which issuitable for a miniature camera. This zoom lens is characterized in thatthe controllable lens group comprises a voltage-controlledelectrowetting device, which device includes a first fluid and a secondfluid having different refractive indices and comprises at least twofirst fluid-second fluid interfaces.

An electrowetting device is a new type of optical component thatincludes two fluids in a fluid chamber, which fluids are in contact viaan interface. Supplying a voltage between two electrodes of the devicecan change the shape of this interface.

A fluid is understood to mean a substance that alters its shape inresponse to any force and that tends to flow or to conform to theoutline of its chamber. Such a fluid may be a gas, a liquid, and amixture of solids and liquids capable of flow.

A lens group may consist of only one lens element, but may alternativelycomprise two or more lens elements. In the embodiments of the zoom lensdescribed hereinafter, the first and second lens groups are eachrepresented by one lens element, but in practice each of these groupsmay comprises more lens elements.

An electrowetting device can be configured as a variable-focus lens. Insuch a lens the fluids have different refractive indices and theirinterface has the shape of a meniscus. Application of a voltage to theelectrodes of the device causes a change in the curvature of therefractive surface formed by the meniscus and thus a change of its lenspower. A zoom lens based on the electrowetting principle and comprisingtwo independently actuated menisci allows zooming, i.e. changing thefocal distance of the zoom lens by adapting the voltage supplied to theelectrodes associated with one of the menisci. Focusing, i.e. keepingthe object in focus for all configurations between Tele and wide, isperformed by adapting the voltage supplied to the electrodes associatedwith the other meniscus.

In this zoom lens system no space needs to be reserved for allowingmovement of lens elements, so that the axial dimension of the system isconsiderably reduced. As motor driven lens elements are no longerrequired, very fast zooming and focusing can be realized and theelectric power for zooming and focusing is considerably reduced, whichmakes the zoom lens system suitable for battery powering.

It is noted that international patent application WO 97/36193 describesa zoom lens system with position-fixed variable-focus lenses. Theselenses are flexible lenses and comprise flexible membranes forming alens chamber, which is filled with a transparent fluid. The curvature ofthe membranes, which form the refractive lens surfaces, can be changed,for example by piezo-electric actuators or by changing the fluid volume.The membranes should be strongly deformed for zooming and focusing. Thedeformations are not directly controlled by a voltage. Moreover, pumpingfluid into and from a flexible lens requires electric power.

It is further noted that patent application publication U.S.2001/0017985 describes an electrowetting optical element capable ofcontrolling its optical transmittance. This element comprises a first,conductive liquid and a second liquid, which have substantially equalrefractive indices but differ in transmittance. The liquids do not mixwith each other and are contained in a sealed container in such a statethat the boundary between them has a predetermined shape. When a voltageis applied between the liquids through electrodes, one of which is incontact with the conductive liquid, the shape of the boundary changes,thereby changing the quantity of transmitted light. The change inboundary shape may also be used to realize a variable-focus lens. Such alens may be used in a zoom lens system wherein the variable focus lensis arranged between a front lens group at and a relay lens group. Thiszoom lens system comprises only one variable-focus element, which isused for zooming, whilst focusing is performed by moving the front lensgroup.

An embodiment of the new zoom lens, which most closely resembles theconventional zoom lens design, has a front lens group at its object sideand a rear lens group at its image side. This embodiment ischaracterized in that the first lens group is the front lens group, thesecond lens group is the rear lens group, and the electrowetting deviceis arranged between the first lens group and the second lens group.

Several embodiments of the zoom lens are possible as regards the numberof interfaces and the distribution of these interfaces overelectrowetting cells.

A first embodiment is characterized in that the electrowetting devicecomprises one electrowetting cell having two first fluid-second fluidinterfaces.

This is the most compact design for an electrowetting device suitablefor a zoom lens.

A second embodiment is characterized in that the electrowetting devicecomprises two electrowetting cells, each having at least one firstfluid-second fluid interface.

This embodiment provides more freedom of design.

The performance of this embodiment can be enhanced considerably if it isfurther characterized in that each electrowetting cell has two firstfluid-second fluid interfaces.

This zoom lens thus comprises four interfaces (menisci), which providesa large freedom of design and allows finer tuning and the use of lowervoltages.

Preferably, the zoom lens is characterized in that the electrowettingdevice comprises a first and a second electrowetting cell, each havingone first fluid-second fluid interface, and in that a lens stop isarranged between the first and the second electrowetting cell.

A similar performance to that of the embodiment having four interfacescan be achieved with this embodiment, but with a reduced number ofinterfaces, thus with a simpler device. Use is made of the fact that thedevice is used more symmetrically, i.e. comparable surface areas of thefirst and second interface are used for refracting the imaging beam. Thestop in a lens system is a diaphragm that restricts the diameter of theimaging beam.

This embodiment having a front lens group at its object side and a rearlens group at its image side may be further characterized in that thefirst lens group is the front group and the second lens group is therear group, and in that the electrowetting device is arranged betweenthe first lens group and the second lens group.

Alternatively, this embodiment may be further characterized in that oneelectrowetting cell forms a front lens group and the secondelectrowetting cell forms a rear lens group, and in that the first lensgroup and the second lens group are arranged between the electrowettingcells, the lens stop being arranged between the first lens group and thesecond lens group.

The succession of fixed and controllable lens groups in this embodimentis totally different from that of a conventional zoom lens.

According to another aspect of the invention, the built-in height of thezoom lens can be further reduced if this lens is further characterizedin that it comprises at least one folding mirror arranged between anelectrowetting cell and one of the first and second lens groups.

This embodiment allows an arrangement of the main portion of a camerawith the zoom lens parallel to the main surface of a device wherein thecamera is to be incorporated.

The minimum built-in size is obtained with an embodiment which ischaracterized in that it comprises two folding mirrors, one at theobject side portion and the other at the image side portion of the zoomlens.

With respect to the possible design of the electrowetting cell(s), thezoom lens is preferably characterized in that an electrowetting cellcomprises:

-   -   a substantially cylindrical chamber having a cylinder wall, the        fluid chamber containing a first fluid and at least one axially        displaced second fluid, the fluids being non-miscible, in        contact across a meniscus interface, and having different        indices of refraction,    -   a fluid contact layer arranged on the inside of the cylinder        wall,    -   a first electrode separated from the first fluid and the at        least one second fluid by the fluid contact layer,    -   a second electrode acting on the second fluid,    -   the fluid contact layer having a wettability by the second fluid        which varies under the application of a voltage between the        first electrode and the second electrode, such that the shape of        the meniscus varies in dependence on the said voltage,    -   wherein the wettability of the fluid contact layer by the second        fluid is substantially equal on both sides of the intersection        of the meniscus with the contact layer when no voltage is        applied between the first and second electrodes.

The equal wettability of the fluid contact layer on both sides of theintersection allows a relatively large movement of the second fluid and,as a consequence, a relative great change in the curvature of themeniscus. This is a large importance for a zoom lens.

In this type of zoom lens the number of electrowetting cells, the numberof interfaces in these cells, and the sequence of the fixed lens groupsand the cells may be as described above herein.

A zoom lens of this type comprising two electrowetting cells may befurther characterized in that the two cells share one fluid chamber.

An embodiment of this type of zoom lens which has a considerably reducedbuilt-in height is characterized in that the fluid chamber comprises atleast one folding mirror, which is formed by a reflective inclinedportion of a fluid chamber wall, which mirror reflects incidentradiation at an angle of substantially 90°.

This embodiment is preferably further characterized in that the fluidchamber comprises two folding mirrors, one at the object side portionand the other at the image side portion of the zoom lens.

This embodiment may be further characterized in that the folding mirrorat the image side portion is integrated with the front lens group.

In this way the number of elements of the zoom lens is reduced by one,which reduces the manufacturing costs.

All of the above-mentioned embodiments may be further characterized inthat the first fluid comprises an insulating liquid and the second fluidcomprises a conducting liquid.

Alternatively, these embodiments may be characterized in that the firstfluid comprises a vapor and the second fluid comprises a conductingliquid.

As a camera wherein the zoom lens is incorporated is distinguished fromconventional cameras by features which are provided by the invention,such a camera forms part of the invention.

As building-in of such a camera in a handheld apparatus provides such anapparatus with a zooming function, such an apparatus also forms part ofthe invention.

These and other aspects of the invention are apparent from and will beelucidated, by way of non-limitative example, with reference to theembodiments described hereinafter. In the drawings:

FIG. 1 shows an electrowetting cell constituting an adjustable lens andthe curvature of the meniscus in the cell when a low voltage is suppliedto its electrodes;

FIG. 2 shows the curvature of the meniscus when an intermediate voltageis supplied;

FIG. 3 shows the curvature of the meniscus when a high voltage issupplied;

FIG. 4 shows an electrowetting cell having two independentlycontrollable menisci and suitable for use in a zoom lens;

FIGS. 5 and 6 show the curvature of and the path of beam rays throughthis cell when in the Tele configuration and in the wide configuration,respectively;

FIG. 7 shows a zoom lens having two electrowetting cells, each havingtwo interfaces;

FIGS. 8 and 9 show the curvature of the menisci of and the paths of beamrays through this zoom lens when in the wide configuration and in theTele configuration, respectively;

FIG. 10 shows a zoom lens having a folding mirror at the front;

FIG. 11 shows this zoom lens also having a folding mirror at the rear;

FIG. 12 shows a zoom lens having an integrated folding mirror and lenselement at the rear;

FIGS. 13 and 14 show the curvature of the menisci of and the paths ofbeam rays through this zoom lens when in the wide configuration and inthe Tele configuration, respectively;

FIGS. 15 and 16 show a first embodiment of a high-performance zoom lenshaving two electrowetting cells, each with one interface, in the wideand Tele configuration, respectively;

FIGS. 17 and 18 show a second embodiment of such a high-performance zoomlens;

FIG. 19 shows a mobile phone provided with a camera which includes azoom lens according to the invention; and

FIG. 20 shows a laptop computer provided with a camera which includes azoom lens according to the invention.

The invention can be best elucidated describing first the principle ofan embodiment of a variable focus lens which is preferably used as avariable lens element in the zoom lens.

FIGS. 1 to 3 show a cross-section of such a lens 1. The lens comprises acylindrical first electrode 2 forming a capillary tube, sealed by meansof a transparent front element 4 and a transparent back element 6 toform a fluid chamber 5 containing two fluids. The electrode 2 may be aconducting coating applied on the inner wall of a tube 7.

In this embodiment the two fluids consist of two non-miscible liquids inthe form of an electrically insulating first liquid A, such as asilicone oil or an alkane, referred to herein further as “the oil”, andan electrically conducting second liquid B, such as an aqueous saltsolution. The two liquids are preferably arranged so as to have equaldensities so that the lens functions independently of orientation, i.e.without dependence on gravitational effects between the two liquids.This may be achieved by an appropriate choice of the first liquidconstituent; for example alkanes or silicone oils may be modified byaddition of molecular constituents to increase their density to matchthat of the salt solution.

Depending on the choice of the oil used, the refractive index of the oilmay vary between 1.25 and 1.60. Likewise, dependent on the amount ofsalt added, the salt solution may vary in refractive index between 1.33and 1.48. The fluids in this embodiment are selected such that the firstfluid A has a higher refractive index than the second fluid B.

The first electrode 2 is a cylinder of inner radius typically between 1mm and 20 mm. The electrode 2 is formed from a metallic material and iscoated with an insulating layer 8, for example of parylene. Theinsulating layer has a thickness of between 50 nm and 100 μm, withtypical values lying between 1 μm and 10 μm.

The insulating layer 8 is coated with a fluid contact layer 10 whichreduces the hysteresis in the contact angle of the meniscus with thecylindrical wall of the fluid chamber. The fluid contact layer ispreferably formed from an amorphous fluorocarbon such as Teflon™ AF1600produced by DuPont™. The fluid contact layer 10 has a thickness ofbetween 5 nm and 50 μm. The AF1600 coating may be produced by repeateddip coating of the electrode 2. A homogeneous layer of material ofsubstantially uniform thickness is formed thereby since the cylindricalsides of the electrode are substantially parallel to the cylindricalelectrode. Dip coating is performed by dipping the electrode whilstmoving the electrode into and out of the dipping solution along itsaxial direction. The parylene coating may be applied by chemical vapordeposition. The wettability of the fluid contact layer 10 by the secondfluid is substantially equal on both sides of the intersection of themeniscus 14 with the contact layer 10 when no voltage is applied betweenthe first and the second electrode.

A second, annular electrode 12 is arranged at one end of the fluidchamber, in this case adjacent the back element 6. At least a portion ofthe second electrode is arranged in the fluid chamber such that theelectrode acts on the second fluid B.

The two fluids A and B are non-miscible so that they tend to separateinto two fluid bodies with a meniscus 14 in between. When no voltage isapplied between the first and second electrode 2 and 12, the fluidcontact layer has a higher wettability with respect to the first fluid Athan the second fluid B. Due to electrowetting, the wettability by thesecond fluid B varies upon application of a voltage between the firstelectrode and the second electrode, which tends to change the contactangle of the meniscus at the three-phase line. The three-phase line isthe line of contact between the fluid contact layer 10 and the twoliquids A and B. The shape of the meniscus is thus variable independence on the applied voltage. The meniscus between the first fluidand the second fluid is called concave if the meniscus is hollow as seenfrom the fluid having the higher refractive index. If this fluid isregarded as a lens, this lens would normally be called concave if themeniscus is concave according to the definition in the previoussentence.

Referring now to FIG. 1, when a low voltage V₁, e.g. between 0 V and 20V, is applied between the electrodes, the meniscus adopts a firstconcave meniscus shape. In this configuration, the initial contact angleθ₁ between the meniscus and the fluid contact layer 10 measured in thefluid B is, for example, approximately 140°. Since the first fluid A hasa higher refractive index than the second fluid B, the lens formed bythe meniscus, here called meniscus lens, has a relatively high negativepower in this configuration. A collimated beam b passing through thelens 1 becomes strongly diverged.

To reduce the concavity of the meniscus shape, a higher voltage isapplied between the first and second electrodes. Referring now to FIG.2, when an intermediate voltage V₂, e.g. 20 V to 150 V, depending on thethickness of the insulating layer 8, is applied between the electrodes,the meniscus adopts a second concave meniscus shape having a radius ofcurvature increased in comparison with the meniscus in FIG. 1. In thisconfiguration the intermediate contact angle θ₂ between the first fluidA and the fluid contact layer 10 is, for example, 100°. Since the firstfluid A has a higher refractive index than the second fluid B, themeniscus lens in this configuration has a relatively low negative power.The collimated beam b becomes weakly diverged.

To produce a convex meniscus shape, a yet higher voltage is appliedbetween the first and second electrodes. Referring now to FIG. 3, when arelatively high voltage V₃, e.g. 150 to 200 V, is applied between theelectrodes, the meniscus adopts a convex shape. In this configuration,the maximum contact angle θ₃ between the first fluid A and the contactlayer 10 is, for example, approximately 60°. Since the first fluid A hasa higher refractive index than the second fluid B, the meniscus lens inthis configuration has a positive power. The lens converts thecollimated beam b into a converged beam.

Note that, whilst the configuration of FIG. 3 can be achieved using arelatively high power, it is preferred in a practical embodiment that adevice comprising the lens as described is adapted to use only low andintermediate voltages in the ranges described. That is to say that thevoltage applied is restricted such that the electric field strength inthe insulating layer is smaller than 20 V/μm, and excessive voltagescausing charging of the fluid contact layer, and hence degradation ofthe fluid contact layer, are not used.

Note furthermore that the initial, low-voltage configuration will varyin dependence on the selection of the fluids (liquids) A and B, independence on their surface tensions. By selecting oil with a highersurface tension, and/or by adding a component, such as ethylene glycol,to the salt solution, which reduces its surface tension, the initialcontact angle can be decreased. In this case the lens may adopt a lowoptical power configuration corresponding to that shown in FIG. 2, andan intermediate power configuration corresponding to that shown in FIG.3. In any case, the lower power configuration remains such that themeniscus is concave, and a relatively wide range of lens powers can beproduced without using an excessive voltage.

Although in the above example the fluid A has a higher refractive indexthan fluid B, the fluid A may also have a lower refractive index thanfluid B. For example, the fluid A may be a (per)fluorinated oil, whichhas a lower refractive index than water. In this case the amorphousfluoropolymer layer is preferably not used, because it may dissolve influorinated oils. An alternative fluid contact layer is e.g. a paraffincoating.

The invention provides a new type of zoom lens by replacing the movablelens elements of a conventional zoom lens by lens elements of the typeshown in FIGS. 1–3. The new type of zoom lens can be made substantiallymore compact and consumes substantially less electric power for the zoomaction and the focusing action than a conventional zoom lens. Theseproperties render the new zoom lens very suitable to be built into aminiature camera for small and/or handheld and/or battery-poweredapparatuses, for example a mobile phone, a personal digital assistant(PDA), a personal computer camera, an intercom system, and an electronicgame.

FIG. 4 shows a cross-section of a possible configuration of thecontrollable lens portion of the new zoom lens, which portion comprisestwo variable-focus lens elements 24, 26 in the form of an electrowettingdevice. This device comprises a cylinder 22 of conductive material. Thecylinder is coated with an insulating layer 28. The inner side of thecylinder is provided with a fluid contact layer 30. The conductivecylinder 22 forms a common first electrode for the lens elements 24 and26. The second electrode of the first lens element 24 is constituted byan annular conductive layer 32 having a central transparent area fortransmitting radiation. A conductive layer 34 at the lower side formsthe second electrode of the second lens element 26. Transparent layers36 and 38 may cover the conductive layers 32 and 34, respectively. Thecentral portion of the cylinder is filled with a first, transparent andnon-conductive liquid or vapor A. At each side of the liquid A, asecond, transparent and conductive liquid B is present, which liquid hasa lower refractive index than the first liquid A. The non-misciblefluids at the upper side are separated by a first meniscus 40, whichforms the first variable-focus lens element. The fluids A and B at thelower side are separated by a second meniscus 42, which forms the secondvariable-focus lens element.

The curvatures of the menisci and thus the focal distances of the lenselements 24 and 26 can be changed independently from each other by meansof controllable voltage sources 44 and 46, respectively. Zooming, i.e.changing the focal distance of the zoom lens, is performed by changingthe meniscus curvature of the first lens element 24 through adaptationof voltage V₁ of source 44. Focusing, i.e. maintaining a sharp image fordifferent zoom configurations, is performed by changing the meniscuscurvature of the second lens element 26 through adaptation of thevoltage V₂ of source 46. Zooming-in means that the focal distance of thezoom lens system is increased and zooming-out means that this distanceis decreased.

FIG. 5 shows the curvature of the menisci and the path of beam rays forthe Tele configuration of an embodiment of the electrowetting devicesimilar to the embodiment of FIG. 4, but with liquids A and Binterchanged. In the Tele configuration, a small object of the scene isimaged on a film or an electronic sensor, for example a CCD or a CMOSsensor. In this configuration of the zoom lens, the fist meniscus 40 hasa convex curvature with a relatively small radius, so that lens element24 acts as a positive, converging lens element having relative highpower. The second meniscus 42 has a concave curvature, so that thesecond lens element 26 acts as a negative, diverging lens element. Thepath of the border rays of the incident beam is shown in FIG. 5 by solidlines. The combination of the two lens elements behaves as a lens havingits second principal point outside and at the object side of the lens,as indicated by the interrupted lines 52. Thus the focal distance of thecombination for the Tele configuration f_(Tele) is relatively great.

FIG. 6 shows the curvature of the menisci and the path of the beam raysfor the wide configuration of the zoom lens. In this configuration, alarger object of the scene is imaged. In the wide position, the convexcurvature of the first meniscus 40 still has a convex curvature, but itsradius is greater than the radius in the Tele configuration. Thecurvature of the second meniscus 42 is now convex so that lens element26 is a positive converging lens. As is shown by the interrupted lines54,56, the second principal point of the combination of lens elements 24and 26 is situated near the second lens element 26. The focal distancefor the wide position f_(wide) is thus relatively small.

FIG. 7 shows an embodiment of a zoom lens system 60 comprising twoelectrowetting lens elements, or cells, 62 and 66 accommodated in thecylindrical structure, which comprises a conductive cylinder, aninsulating layer, a fluid contact layer, and second electrodes asdescribed above. FIG. 7 only shows the cylinder 70. The cylinder isclosed at the object side by a front group in the form of a solid lenselement 72 and at the image side by a rear group in the form of a solidlens element 74. The front element 72 is a positive, convex-convex lenselement of highly refractive plastic, such as polycarbonate (PC) orcyclic olefin polymer (COC), and provides desired initial focusingcharacteristics. This lens element may have at least one asphericalsurface to correct for spherical aberrations of the zoom lens. Lenselement 12 is followed by a first electrowetting cell 62 having liquidB/ liquid A/ liquid B interfaces. This cell may be sealed by a flatplate 78 of transparent plastic, such as PC or COC, which comprises thefield stop 80 of the zoom lens. The plate 78 seals the front side of asecond electrowetting cell 66 of liquid B/ liquid A/ liquid Binterfaces. The second electrowetting cell is sealed by the rear lenselement 74. Lens element 74 is a positive, convex-convex element ofhighly refractive plastic, such as PC or COC. This lens element may beused as a field flattener, and at least one of its refractive surfacesmay be an aspherical surface. Liquid A of the electrowetting cells maybe a (per)fluorinated oil having a refractive index of 1.536 and liquidB may be an aqueous solution of salt having a refractive index of 1.336.The refractive index of COC is 1.536.

Each electrowetting cell contains two liquid interfaces or menisci 63,64and 67,68, respectively, which allows changing of both refractivesurfaces of the electrowetting lens element. The lens power changerequired for zooming or focusing can be distributed over the tworefractive surfaces so that per surface a smaller change is needed. Thismeans that the required power change can be realized with lower voltagesthan those needed for an electrowetting lens element having only oneliquid interface. The two electrowetting cells 62 and 64 of the zoomlens may also be replaced by the electrowetting lenses 24 and 26 of FIG.4.

FIGS. 8 and 9 show ray-trace plots of an embodiment of the zoom lenswhich is similar to that shown in FIG. 7, but with a concave-convexfront lens element 72′ and a convex-planar rear lens element 74′. FIG. 8shows the wide configuration and FIG. 9 the Tele configuration. It isimmediately clear from FIGS. 8 and 9 that both interfaces 63 and 64change for zooming and that both interfaces 66 and 68 change forfocusing.

A practical embodiment of the zoom lens of FIGS. 8 and 9 shows thefollowing characteristics:

Tele wide Focal length 7.11 mm 3.35 mm F/number 3.4 2.6 Angle of viewdiagonal 28° 56°This zoom lens is suitable for co-operating with a CMOS sensor 48 of theVGAS type having 640×480 pixels and a pixel size of 4.2 μm.

A zoom lens comprising electrowetting cells instead of solid lenselements has high zoom (and focusing) speed, is directly electricallyactuated, has a small size, and can be mass-manufactured at low cost.These properties make the electrowetting zoom lens very suitable for usein a miniature camera to be incorporated in several types of apparatus,especially handheld and battery-powered apparatuses.

A zoom lens for a miniature camera to be built into a handheldapparatus, such as a mobile phone, should have a built-in height assmall as possible. As the front lens element of such a camera willusually be accommodated in the front surface of the apparatus (thesurface at which the user looks), the built-in height of a zoom lensdiscussed so far will be determined by the axial length of the zoomlens. This means that the built-in height of the camera, which height ismainly determined by the length of the zoom lens, should fit into thedepth of the apparatus, which depth is preferably as small as possible.

According to a further aspect of the invention, the built-in height ofthe zoom lens can be substantially decreased by including a foldingmirror in the zoom lens. FIG. 10 shows a zoom lens 90 comprising such amirror 92, which forms part of the wall of the first electrowetting cell62. The zoom lens of FIG. 10 comprises the same elements as the zoomlens of FIG. 7, which elements are denoted by the same referencenumerals. The light beam b from the object scene is incidentperpendicularly on the front lens element 72. After having passed theinterfaces 63 and 64 of the first electrowetting cell 62, the beam isreflected by the mirror 92 in the horizontal direction if this mirror isarranged at an angle of 45° with respect to the chief ray of the beam b.The other elements of the zoom lens: the transparent plate 78 with thestop 80, the second electrowetting cell 66, the rear lens element 74,and the sensor 48 are arranged in the horizontal direction. Thishorizontal direction is parallel to the front surface of the apparatus,wherein the camera, i.e. the zoom lens and the sensor 48, should bebuilt in. In this way the built-in height h_(c) of the camera is reducedto the sum of the height of the mirror and the geometrical length of thelight path from the second interface 64 to the outer surface of thefront lens element 72.

The mirror is preferably arranged in a first position close to the frontlens element 72 where sufficient space is available for this mirror. Inthe design of FIG. 7, wherein the first interface 63 is close to thefront lens, this position is immediately behind the second interface. Inanother design of the zoom lens, the folding mirror could be arranged inanother position, provided that this position is as close as possible tothe front lens.

The inclusion of a second folding mirror at the rear side of the zoomlens allows a further reduction of the total size of the camera. FIG. 11shows a zoom lens 100 comprising such a mirror 94. This mirror reflectsthe beam b, which has passed the electrowetting cell 66, in the verticaldirection so that the rear lens element 74 and the sensor 48 can bearranged in this direction. In this way the dimension of the camera inthe horizontal direction is reduced, without increasing its built-inheight.

As is shown in FIG. 12, the folding mirror can be integrated with therear lens element into a mirror lens element 96. This mirror lenselement has a reflective flat base surface 97 arranged at an angle of,for example, 45° with respect to the chief ray of the beam b, and twocurved surfaces 98 and 99 which form the refractive surfaces of the lenselement. In this way the number of elements of the zoom lens and thusits manufacturing costs can be reduced. At least one of the curvedsurfaces of the element 95 may be an aspherical surface, as is the casefor the rear lens element 74 in FIGS. 7, 10 and 11.

FIGS. 13 and 14 show ray-trace plots of an embodiment of the zoom lenssimilar to the embodiment of FIG. 12, but with a concave-convex frontlens element 72″ and a convex-concave rear lens element 96′, instead ofdouble convex lens elements. FIG. 13 shows the wide configuration andFIG. 14 the Tele configuration. It is immediately clear from theseFigures., that both interfaces 63 and 64 change for zooming and thatboth interfaces 67 and 68 change for focusing.

The embodiments of the zoom lens shown in FIGS. 7–14, wherein theelectrowetting device has two electrowetting cells each comprising twofirst liquid-second liquid interfaces, has an excellent performance.According to a further inventive step, the number of interfaces ormenisci can be reduced from four to two while maintaining the high-levelperformance. Such a reduction of the number of menisci means asubstantial simplification of the electrowetting device in terms ofconstruction and of control of the curvatures of the menisci. Theelectrowetting device according to the improved design comprises twoelectrowetting cells each having one meniscus. Essential for theimproved design is that the electrowetting cells are arranged atdifferent sides of the lens stop. The stop of a lens system is adiaphragm that restricts the diameter of the imaging beam and preventsstray radiation or radiation from unwanted reflections from beingintroduced into the imaging beam and causing a reduction in the imagecontrast. Such a diaphragm ensures that the beam diameters are the samefor all imaging beam portions, so that the illumination intensity andthe resolution are constant in the image field. The effect of arrangingthe lens stop between the electrowetting cells is that the twointerfaces are used symmetrically, i.e. the central surface areas of thefirst and second interfaces covered by the imaging beam haveapproximately the same size. A second interface is then no longer neededin the cells.

FIGS. 15 and 16 shows a ray-trace plot in the wide configuration and inthe Tele configuration of a first embodiment 110 of the high-performancezoom lens having only two interfaces. The lens comprises a fixed frontlens comprising, from the object side (left) to the image side (right);a fixed front lens element 112, a first electrowetting cell 120, asecond electrowetting cell, and a fixed rear lens element 114. The firstelectrowetting cell 120 comprises a chamber 132 sealed by a front plate126 and a rear wall 128 for holding liquids A and B that have aninterface in the form of a meniscus 124. The second electrowetting cell130 comprises a chamber 132 sealed by a rear plate 136 and a front wall138 for holding liquids A and B that have an interface in the form of ameniscus 134. The stop 116 of the zoom lens is situated between thecells 120, 130. In the wide configuration of the zoom lens, the meniscus124 is concave and the first cell 120 thus forms a concave lens element,whereas the meniscus 134 is convex and the second cell 130 thus forms aconvex lens element. In the Tele configuration, the meniscus 124 isconvex and the first cell 120 thus forms a convex lens element, whereasthe meniscus 134 is concave and the second cell 130 thus forms a concavelens element.

A practical embodiment of the zoom lens of FIGS. 15 and 16 shows thefollowing characteristics:

Tele wide Focal length 7.03 mm 3.61 mm F/number 3.4 2.6 Angle of viewdiagonal 28° 56°

FIGS. 17 and 18 shows a ray-trace plot in the wide configuration and inthe Tele configuration, respectively, of a second embodiment 140 of thehigh-performance zoom lens having only two menisci. The firstelectrowetting cell 150 is now arranged at the front side of the zoomlens and the second electrowetting cell at the rear end. The first andsecond lens elements 142 and 144 are positioned between the cells 150and 160. The lens stop 146 is now between the lens elements 142 and 144.The first electrowetting cell 150 comprises a chamber 152 sealed by afront plate 156 and a rear plate 158 for holding the two liquids A and Bthat have an interface in the form of a meniscus 154. The secondelectrowetting cell 160 comprises a chamber 162 sealed by a front plate168 and a rear plate 166 for holding liquids A and B that have aninterface in the form of a meniscus 164. As is immediately clear fromFIGS. 17 and 18, switching from the wide configuration to the Teleconfiguration is again performed by reversing, and adapting the radiiof, the curvatures of menisci 154 and 164 by supplying appropriatevoltages to the electrodes of the cells as described above.

A practical embodiment of the zoom lens of FIGS. 17 and 18 shows thefollowing characteristics:

Tele wide Focal length 8.98 mm 4.79 mm F/number 3.5 2.8 Angle of viewdiagonal 28° 56°

If so desired, the zoom lens of FIGS. 15, 16 and the zoom lens of FIGS.17, 18 may also be provided with one or two folding mirrors to shortenthe built-in height of these lenses. The first folding mirror will bearranged between front element 112 and the first cell 120 in theembodiment of FIGS. 15, 16 and between the first cell 150 and the firstfixed lens element 142 in the embodiment of FIGS. 17, 18. The secondfolding mirror will be arranged between the second cell 130 and the rearlens element 114 in the embodiment of FIGS. 15, 16 and between thesecond fixed lens element 144 and the second cell 160 in the embodimentof FIGS. 17, 18.

For some applications of the zoom lens, where even finer tuning of thelens is required, a movable lens group (one or more lens elements)controlled by a motor or otherwise may be included in the zoom lens. Theelectrowetting device then still provides the same advantages as in thezoom lens described above.

FIG. 19 shows an example of a handheld apparatus wherein the zoom lensof the invention can be used. The apparatus is a mobile phone 170 shownin front view in FIG. 19. The mobile phone has a microphone 172 whichinputs the user's voice as data, a loudspeaker 174 which outputs thevoice of a communicating partner, and an antenna 176 which transmits andreceives the communicating waves. The mobile phone further comprises aninput dial 178 by means of which the user inputs data, such as a phonenumber to be dialed, and a display 180, for example a liquid crystaldisplay panel. This panel may be used to display a photograph of thecommunicating partner or of the user, or to display data and graphics. Adata processing unit (not shown) is included in the mobile phone forprocessing the input data and the received data.

The phone 170 is provided with a miniature camera 182 comprising a zoomlens as described above for photographing a scene, graphics, or data tobe communicated to the partner or the user. Of this camera only theentrance surface 184 of the first lens element of the zoom lens isvisible. This element may be the front lens element 24, 72, or 112 asshown in FIG. 4, in FIGS. 7, 8, 10, 11, 12, 13, and in FIG. 15,respectively, or the first electrowetting cell 150 shown in FIG. 17. Theother elements of the camera, i.e. the two electrowetting cells and therear lens or, for the embodiment of FIG. 17, the two fixed lens elementsand the second electrowetting cell, and the sensor may be arranged alonga line perpendicular to the front surface of the phone, i.e. in thedirection perpendicular to the plane of drawing of FIG. 19 if thedimension of the phone in this direction is large enough. Preferably,the zoom lens comprises at least one folding mirror. Then at least thetwo electrowetting cells or, for the embodiment of FIG. 17, at least thetwo fixed lens elements are arranged along a line which is parallel tothe front surface of the phone, which may then be relatively thin.

Another handheld apparatus wherein the invention may be implemented is apersonal digital assistant (PDA) provided with a miniature camera. Sucha camera with a zoom lens as described above may be arranged in the PDAin the same way as described for the mobile phone.

FIG. 20 shows a laptop computer (notebook) as an example of a portableapparatus wherein the invention may be implemented. The laptop computer190 comprises a base portion 192 into which a keyboard 195 and theprocessor unit are incorporated. A cover portion 196, which can berotated with respect to the base portion, includes a display 198 and aminiature camera 200. Such a camera provided with a zoom lens asdescribed above may be arranged in the laptop in the same way asdescribed for the mobile phone.

The invention may be used not only in a camera for a handheld apparatus,like a mobile phone, a digital personal assistant, a pocket computer,and an electronic toy, or for a portable apparatus, but also in othertypes of built-in cameras. The invention may also be used innon-built-in cameras, like cameras for desktop computers, cameras forintercom systems, and pocket-sized and other-size cameras, for exampledigital cameras. The camera may be a still-picture (photo) camera or avideo camera. It is irrelevant for the invention whether the camera usesa film or an electronic sensor, for example a CCD sensor or CMOS sensor.

1. A zoom lens having at least a first fixed lens group, a second fixedlens group, and a controllable lens group, characterized in that thecontrollable lens group comprises a voltage-controlled electrowettingdevice, which device includes a first fluid and a second fluid havingdifferent refractive indices and comprises at least two firstfluid-second fluid interfaces.
 2. A zoom lens as claimed in claim 1having a front lens group at its object side and a rear lens group atits image side, characterized in that the first lens group is the frontlens group, the second lens group is the rear lens group, and theelectrowetting device is arranged between the first lens group and thesecond lens group.
 3. A zoom lens as claimed in claim 2, characterizedin that the electrowetting device comprises one electrowetting cellhaving two first fluid-second fluid interfaces.
 4. A zoom lens asclaimed in claim 2, characterized in that the electrowetting devicecomprises a first electrowetting cell and a second electrowetting celleach having at least one first fluid-second fluid interface.
 5. A zoomlens as claimed in claim 4, characterized in that each electrowettingcell has two first fluid-second fluid interfaces.
 6. A zoom lens asclaimed in claim 1, characterized in that the electrowetting devicecomprises a first and a second electrowetting cell each having one firstfluid-second fluid interface, and in that a lens stop is arrangedbetween the first and the second electrowetting cell.
 7. A zoom lens asclaimed in claim 6 having a front lens group at its object side and arear lens group at its image side, characterized in that the first lensgroup is the front lens group and the second lens group is the rear lensgroup, and in that the electrowetting device is arranged between thefirst lens group and the second lens group.
 8. A zoom lens as claimed inclaim 6, characterized in that one electrowetting cell forms a frontgroup and the second electrowetting cell forms the rear group, and inthat the first lens group and the second lens group are arranged betweenthe electrowetting cells, the lens stop being arranged between the firstlens group and the second lens group.
 9. A zoom lens as claimed in claim1, characterized in that it comprises at least one folding mirrorarranged between an electrowetting cell and one of the first and secondlens groups.
 10. A zoom lens as claimed in claim 9, characterized inthat it comprises two folding mirrors, one at the object side portionand the other at the image side portion of the zoom lens.
 11. A zoomlens as claimed in claim 1, characterized in that an electrowetting cellcomprises: a substantially cylindrical chamber having a cylinder wall,the fluid chamber containing a first fluid and an axially displacedsecond fluid, the fluids being non-miscible, in contact across ameniscus interface and having different indices of refraction, a fluidcontact layer arranged on the inside of the cylinder wall, a firstelectrode separated from the first fluid and the second fluid by thefluid contact layer, a second electrode acting on the second fluid, thefluid contact layer having a wettability by the second fluid whichvaries under the application of a voltage between the first electrodeand the second electrode, such that the shape of the meniscus varies independence on the said voltage, wherein the wettability of the fluidcontact layer by the second fluid is substantially equal on both sidesof the intersection of the meniscus with the contact layer when novoltage is applied between the first and second electrodes.
 12. A zoomlens as claimed in claim 11 comprising one electrowetting cell,characterized in that a second fluid is present on either side of thefirst fluid, the first fluid and the second fluids being in contactacross respective first and second meniscus interfaces.
 13. A zoom lensas claimed in claim 12, characterized in that a folding mirror isarranged between the front lens group and the electrowetting cell.
 14. Azoom lens as claimed in claim 12, characterized in that a folding mirroris arranged between the electrowetting cell and the rear lens group. 15.A zoom lens as claimed in claim 11 comprising two electrowetting cells,characterized in that each cell has one meniscus interface and in that alens stop is arranged between the cells.
 16. A zoom lens as claimed inclaim 11, characterized in that a second fluid is present on either sideof the first fluid in each electrowetting cell, the first fluid and thesecond fluids being in contact across respective first and secondmeniscus interfaces.
 17. A zoom lens as claimed in claim 15,characterized in that the two cells share one fluid chamber.
 18. A zoomlens as claimed in claim 17, characterized in that the fluid chambercomprises at least one folding mirror formed by a reflective inclinedportion of a fluid chamber wall, which mirror reflects incidentradiation at an angle of substantially 90°.
 19. A zoom lens as claimedin claim 18, characterized in that the fluid chamber comprises twofolding mirrors, one at the object-side portion and the other at theimage side portion of the zoom lens.
 20. A zoom lens as claimed in claim13, characterized in that the folding mirror at the image-side portionis integrated with the front lens group.
 21. A zoom lens as claimed inclaim 1, characterized in that the first fluid comprises an insulatingliquid and the second fluid comprises a conducting liquid.
 22. A zoomlens as claimed in claim 1, characterized in that the first fluidcomprises a vapor and the second fluid comprises a conducting liquid.23. A camera comprising a zoom lens, wherein the zoom lens comprises atleast a first fixed lens group, a second fixed lens group, and acontrollable lens group, characterized in that the controllable lensgroup comprises a voltage-controlled electrowetting device, which deviceincludes a first fluid and a second fluid having different refractiveindices and comprises at least two first fluid-second fluid interfaces.24. A handheld apparatus comprising a camera, the camera comprising azoom lens, wherein the zoom lens comprises at least a first fixed lensgroup, a second fixed lens group, and a controllable lens group,characterized in that the controllable lens group comprises avoltage-controlled electrowetting device, which device includes a firstfluid and a second fluid having different refractive indices andcomprises at least two first fluid-second fluid interfaces.